# Imaginary problem with speed control on Jrk

Hi,

I’m having a problem to get frequency feedback to work. I upgraded to v.1.3 firmware. I’m using Jrk Configuration Utility and open loop speed control works fine, but frequency feedback mode works just like open loop. I’m reading a stable frequency from encoder, PID is set to (1, 0, 0). Here is a test setting: target = 2403, scaled feedback = 2175, duty cycle = 38%, error = -7% and these values remain stable, no speed adjustment at all. What I’m I doing wrong here?

Thank you,
Paul.

Hello,

From your numbers, it sounds like your jrk is working just fine. With a target of 2403 and a scaled feedback of 2175, your error is 228. Using PID of 1,0,0, this translates directly into a duty cycle of 228, or 38%.

I would suggest increasing the proportional term or adding an integral term if you want to get stronger feedback.

By the way, you can’t say that frequency feedback works “just like” open loop control, since with open loop, a target of 2403 would give you a duty cycle of 100%, right?

-Paul

HI Paul,

I wasn’t suggesting that Jrk is not working well, I was hinting that my understanding how it should behave was off. I was working under assumption that I can set a target speed and controller will produce such duty cycle that feedback matches target speed. Looking at PID equation (proportional term only, P != 0):

Duty Cycle = P*(Feedback-Target)

Target = Feedback is achievable only for Duty Cycle = 0, but I want to achieve non-stationary, constant speed. It seems that the PID control strategy is geared toward position control, but not speed control. Shouldn’t there be another term proportional to target speed? I apologize, if this sounds too basic, but I’m a little bit puzzled by this issue.

Thanks,
Paul.

OK, I have it working. The integral term has to be non-zero in order to achieve non-stationary target speed. The PID thingy is not exactly what I thought it was. Case closed.

Paul.

Yup, the integral term is necessary to get the speed to reach the target, since there is constantly resistance trying to slow the motor down. It’s a similar situation to doing position control lifting a large load, when the load is constantly pulling the motor away from the target point. That is what the integral term is for!

-Paul